The present invention relates to random copolymers of propylene with other 1-alkenes having up to 10 carbon atoms,
whose content of comonomers is in the range from 0.7 to 1.4% by weight if the only comonomer present in the propylene copolymers is ethylene, or
whose content of comonomers is in the range from 0.7 to 3.0% by weight if at least one C4–C10-1-alkene is present as comonomer, and
whose cold-xylene-soluble fraction is from 1.0 to 2.5% by weight if ethylene is present as a comonomer in the propylene copolymers, or
whose cold-xylene-soluble fraction is from 0.75 to 2.0% by weight if the only comonomers present are C4–C10-1-alkenes.
The present invention also relates to a process for preparing the random copolymers of propylene, to their use for producing films, fibers or moldings, to the films, fibers and moldings themselves, and also to biaxially stretched films made from such random copolymers of propylene and to processes for their production.
Propylene homopolymers and copolymers of propylene and other 1-alkenes are widely applied in the production of films, fibers and moldings from thermoplastic molding compositions. For brevity, both the propylene homopolymers and the propylene copolymers are frequently referred to in such cases as polypropylene. The polymers used for producing films, in particular biaxially stretched films, are in particular homopolymers or random copolymers of propylene. The biaxially stretched films produced from these materials are often also termed BOPP (biaxially oriented polypropylene) films.
However, when producing biaxially polypropylene oriented films it is not only desirable that the resultant films have good mechanical and service properties, such as stiffness, toughness, transparency or gloss; good processability is also a prime requirement. Good processability results firstly in a trouble-free production process. This is desirable since every interruption of continuous production (e.g. a film break-off) requires a time-consuming restart of production. Secondly, better film quality is obtained, for example more uniform thickness distribution. Good processability also means that the production process is not excessively sensitive to temperature variations, i.e. that a large amount of processing latitude is available.
Since the processability of conventional propylene homopolymers is not adequate to allow their use in the production of biaxially stretched films, use is frequently made of propylene homopolymers having increased soluble fractions. However, films produced from these materials have disadvantages in stiffness, and their processability is still not satisfactory.
Conventional random propylene copolymers, as described, for example, in EP-A 778 295, are unsuitable for producing BOPP films, since their stiffness would be far too low. In addition, their high soluble fractions makes it impossible to use the films in the food packaging sector.
EP-A 339 804 describes reactor blends made from a propylene homopolymer and a random propylene copolymer and particularly suitable for producing oriented films. However, such reactor blends can be obtained only via a reactor cascade and are therefore unattractive on cost grounds.
To improve extensibility, U.S. Pat. No. 4,355,144 proposes the use of a random propylene-ethylene copolymer with from 0.1 to 1.0% by weight of ethylene, polymerized with a titanium trichloride-containing catalyst. However, even with very small amounts of ethylene the isotacticity of the copolymer is drastically reduced, and the stretched products obtained have low stiffness and high extractable fractions.
EP-A 115 940 discloses propylene-ethylene copolymers suitable for producing biaxially stretched films and having from 0.1 to 2.0 mol % of ethylene and high isotacticity. The isotacticity is determined as triad tacticity in the 13C NMR spectrum. Although these copolymers have good extensibility, stiffness, transparency, impact strength and resistance to heat shrinkage, their mechanical, optical and Theological properties are frequently inadequate for the requirements of BOPP film manufacturers. In particular, it is frequently the case that they do not have ideal processability at the same time as very good mechanical properties.